Problem: Solve for $r$ : $17 = 8 + r$
Explanation: Subtract $8$ from both sides: $ \begin{eqnarray} \\ \hphantom{17 {- 8}} &\hphantom{=}& \hphantom{r} \\ 17 &=& 8 + r \\ \\ {-8} && {-8} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 17 {- 8} &=& r \\ \end{eqnarray} $ Simplify. $ r = 9$